golang recursive function calling itself as a goroutine doesn't work as expected

Issue

This recursive function works as expected (returns 5 lines with numbers 5 to 1):

package main
import (
    "fmt"
)
func recur(iter int) {
    if iter <= 0 {
        return
    }
    fmt.Println(iter)
    recur(iter-1)
}
func main() {
    recur(5)
}

this one does not (returns only 1 line with number 5):

    package main
import (
    "fmt"
)
func recur(iter int) {
    if iter <= 0 {
        return
    }
    fmt.Println(iter)
    go recur(iter-1)
}
func main() {
    recur(5)
}

The difference is that in the second implementation, function calls itself as a goroutine. (line go recur(iter-1) )

So can someone explain this behaviour?

Solution

If you make everything asynchronous, there is nothing in main to wait on. You have to explicitly wait on the go routines so your program does not exit before the recursive process finishes.

Use sync.WaitGroup or something similar for synchronization. Example (On Play):

func recur(iter int, g *sync.WaitGroup) {
    defer g.Done()
    if iter <= 0 {
        return
    }
    fmt.Println(iter)
    go recur(iter-1, g)
}

func main() {
    g := &sync.WaitGroup{}
    runs := 5
    g.Add(runs)
    recur(runs, g)
    g.Wait()
}

Answered By – nemo

Answer Checked By – Jay B. (GoLangFix Admin)

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